WATERBURY — A proposed five-story, 73-room hotel on the site of the Thatcher Brook Inn on Route 100 doesn’t fit the character of the existing historic district, according to state officials at an Act 250 hearing on the project last week, an assessment that could derail the proposal unless design changes are made.

The property lies in the Colbyville Historic District, which includes the Colbyville mansion and other buildings. Devin Colman, historic preservation review coordinator with the state Division for Historic Preservation, said the existing buildings in the district are two and a half stories at most.

“The overall context is of an historic, residential grouping of buildings, and this is a five-story hotel in the middle of that district,” Colman said at the hearing. “We don’t save an historic district by dropping (a) five-story hotel in the middle of it.”

Property owner Joe Larkin of Larkin Realty in Burlington is seeking to remodel and expand the Thatcher Brook Inn. It would become a new franchise of the Marriott Fairfield Inn and Suites chain.

At the hearing, Act 250 Region 5 commission member Jerry Diamond, one of three panelists reviewing the project, chastised the developers for not reconciling the size of the project with the standards of the Division for Historic Preservation, which is part of the Agency of Commerce and Community Development.

“I understand ... your client wants 73 rooms. But I travel for a living 80 to 100 days a year. I stay in Fairfield Inns across this country, and I stay in plenty of them that are only two and three stories tall,” Diamond said at the hearing, held in a conference room of the Best Western Plus Hotel on Route 100. “It’s a shame that was not negotiated with Historic Preservation and had to come to us without that agreement.”

Colman said the developers presented a take-it-or-leave-it approach, but Larkin disputes that.

While the construction of the hotel would demolish parts of two historic buildings and a walkway between them, the core of the historic structures would remain intact. The development team had considered moving or demolishing some of the buildings but decided the five-story design that integrated the two cores of the historic buildings was the best approach, according to Greg Rabideau, the project architect.

In seeking to mitigate the project’s impact, the development team also moved the design farther back onto the property away from the road.

The development team contended there wasn’t as much dialogue as it would have liked with the Division for Historic Preservation, suggesting that state officials were challenged by a significant workload in connection with Tropical Storm Irene and the destruction it caused at the Waterbury state office complex.

The development team also has argued that the project would not hurt the character of the area. In a formal October filing, Rabideau’s firm pointed to the nearby Shaw’s development and the size of the Ben & Jerry’s factory, as well as other development on Route 100 including gas stations and a shopping center.

“I think it’s unfair to the applicant to sort of push back and say that this is some pristine historic village that needs to be preserved because the buildings are compromised, the surroundings are compromised,” Rabideau said.

The height of the hotel remains the central sticking point.

Colman declined to specify at last week’s hearing, and in an interview afterward, exactly what height might be acceptable.

Diamond said, “Everything ... can be resolved, signage on down, except for the size of the project. You’ve made it much more difficult for us.”

“And ultimately if we decide to go with Historic Preservation’s argument, ” he said, “we’re going to put you back with them anyway if you want to continue on the site. You’re going to ultimately have to negotiate a size that they can live with.”

Larkin, the developer, said after the meeting that he imagines some significant redesigning would occur to address the state’s concerns and make the project happen.